Where does the gold come from? New perspectives on the synthesis of elements in the universe – sciencedaily


How are the chemical elements produced in our Universe? Where do heavy elements like gold and uranium come from? Using computer simulations, a research team from the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, in collaboration with colleagues from Belgium and Japan, shows that the synthesis of heavy elements is typical of some black holes with accumulations of matter in orbit, called accretion discs. The predicted abundance of formed elements provides insight into the heavy elements that need to be investigated in future laboratories – such as the Antiproton and Ion Research Facility (FAIR), currently under construction – to unravel the origin of the elements. heavy. The results are published in the journal Monthly notices from the Royal Astronomical Society.

All heavy elements on Earth today were formed under extreme conditions in astrophysical environments: inside stars, in stellar explosions, and in the collision of neutron stars. Researchers are intrigued by the question in which of these astrophysical events exist the appropriate conditions for the formation of the heavier elements, such as gold or uranium. The first spectacular observation of gravitational waves and electromagnetic radiation from a neutron star fusion in 2017 suggested that many heavy elements can be produced and released in these cosmic collisions. However, the question remains open as to when and why the material is ejected and whether there may be other scenarios in which heavy elements can be produced.

Promising candidates for the production of heavy elements are black holes orbiting by an accretion disk of dense, hot matter. Such a system is formed both after the merger of two massive neutron stars and during a so-called collapsar, the collapse and subsequent explosion of a rotating star. The internal composition of such accretion disks has so far not been well understood, in particular as regards the conditions under which an excess of neutrons is formed. High neutron count is a basic requirement for heavy element synthesis because it enables the rapid neutron capture process or r process. Nearly massless neutrinos play a key role in this process, as they enable the conversion between protons and neutrons.

“In our study, we systematically studied for the first time the conversion rates of neutrons and protons for a large number of disk configurations using elaborate computer simulations, and we found that the disks are very rich in both neutrons. that certain conditions are met, ”says Dr. Oliver Just of the Relativistic Astrophysics group of the Theory research division of GSI.“ The deciding factor is the total mass of the disc. The more massive the disc, the more neutrons are formed from protons by electron capture under neutrino emission, and are available for heavy element synthesis by means of r- However, if the mass of the disc is too high , the reverse reaction plays an increased role so that more neutrinos are recaptured by the neutrons before they leave the disk. These neutrons are then converted back to protons, which hinders the r process. “As the study shows, the optimum disk mass for prolific heavy element production is around 0.01 to 0.1 solar mass. The result provides strong evidence that neutron star mergers producing accreting disks with these exact masses could be the point of origin for much of the heavy elements. However, if and how often such disks of accretion to occur in collapsar systems is currently unclear.

In addition to the possible processes of mass ejection, the research group led by Dr Andreas Bauswein is also studying the light signals generated by the ejected matter, which will be used to infer the mass and composition of the ejected matter in future observations. of neutron star collision. An important element in correctly reading these light signals is an accurate knowledge of the masses and other properties of the newly formed elements. “These data are currently insufficient. But with the next generation of accelerators, like FAIR, it will be possible to measure them with unprecedented precision in the future. The well-coordinated interaction of theoretical models, experiments and astronomical observations We will allow researchers in the coming years to test neutron star mergers as the origin of r process elements, ”Bauswein predicts.

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